In connection with an operational special situation of an elevator, such as in connection with an emergency stop of the elevator, on service drive, on rescue drive, in connection with an electricity outage and in connection with accurate releveling of the elevator, the elevator is generally stopped by starting a process which activates a machinery brake and also disconnects the current supply to the elevator motor. A quick stop situation of an elevator begins when the aforementioned process starts and ends when the elevator has stopped.
A quick stop situation of an elevator typically starts when the safety circuit of the elevator opens. Opening of the safety circuit starts disconnection of the electricity supply of the electromagnet of the machinery brake. When the electricity supply is disconnected and when the machinery brake activates, the brake shoe engages mechanically against the braking surface of the hoisting machine to brake the movement of the traction sheave of the hoisting machine. The elevator car is suspended in the elevator hoistway with ropes passing via the traction sheave, so that the brake shoe at the same time also brakes the movement of the elevator car. Opening of the safety circuit also causes opening of the contactor, or other controllable switch, fitted into the power supply circuit of the elevator motor, such that the electricity supply from the electricity network to the elevator motor disconnects.
The solution presented above contains problems. Engaging the brake shoe against the braking surface of the hoisting machine normally takes rather a long time, up to several hundred milliseconds; this is a consequence of, inter alia, the effect of the inductance of the electromagnet of the brake. Disconnection of the electricity supply of the elevator motor, on the other hand, causes discontinuation of the braking effect of the current of the elevator motor on the movement of the elevator car. If in this case the electricity supply to the elevator motor ceases before the machinery brake has been activated, the elevator car goes into a state of free fall for a moment, in which state the movement of the traction sheave of the hoisting machine/elevator car is not braked in any way whatsoever. The problem is accentuated in elevators comprising a gearless hoisting machine, because in this case the force difference in the suspension ropes acting on the different sides of the traction sheave of the hoisting machine generally produces greater acceleration during free fall of the elevator car than when using a geared hoisting machine. A state of free fall makes e.g. evaluation of the stopping point of the elevator car difficult. A state of free fall can, inter alia, cause the elevator car in certain operating situations, such as in connection with accurate releveling, to fail to stop sufficient accurately at a floor level. In this case a sill remains between the floor of the elevator car and the floor level, which sill hampers moving between the elevator car and the floor level. The sill can also cause a dangerous situation, e.g. to a person using a wheelchair, when moving a heavy object over the sill on rollers, et cetera.
Efforts have been made to speed up activation of the machinery brake in order to solve the problem by adding electronic/electrical components, such as resistors, capacitors or varistors, to the electricity supply circuit of the electromagnet of the machinery brake, with which components the quenching voltage over the coil of the electromagnet is increased, and thus reduction of the current is speeded up in a current disconnection situation. The use of a large quenching voltage, however, wears the components of the electricity supply circuit of the electromagnet. As the quenching voltage increases, many components also heat up more, causing, in addition to the lifetime problems of the components, also e.g. problems relating to thermal dimensioning. Increasing the quenching voltage also often causes the noise of the brake to increase when the brake shoe hits against the braking surface.
Efforts have been made to solve the problem also by increasing the disconnection delay of the current supply of the elevator motor, which disconnection delay lasts from the opening of the safety circuit to when the electricity supply to the elevator motor disconnects. The delay can be increased e.g. by adding a special delay circuit in connection with the control coil of the contactor that disconnects the electricity supply to the elevator motor. From this, however, it follows that disconnection of the current supply of the elevator motor slows down in all operating situations. In certain situations, such as when detecting a fault in the power supply circuit of the elevator motor, the current supply to the elevator motor must be disconnected as quickly as possible in order to prevent a hazardous situation. An extra disconnection delay might slow down use of the elevator also in connection with normal operation of the elevator. Therefore, instead of solving the problems, a delay circuit in fact creates new problem situations.